This invention relates to apparatus for the cryofixation of specimens particularly biological, medical or technical specimens having a high water or fluid content.
Two methods are currently favoured in the cryofixation (rapid freezing) of specimens having a high liquid content, such as biological or medical specimens, gels and suspensions, for photo-examination, particularly for electron microscope examinations. These are immersion cryofixation, in which the specimen is immersed in a cooling bath, and metal mirror cryofixation, in which the specimen is applied against a highly polished metal surface. In both cases it is important to withdraw the greatest possible quantity of heat from the specimen within the shortest time, in order that any segregation of the aqueous or liquid mixture phases is reduced to a minimum.
In this respect, neither of these two methods is ideal for all known specimens and for all known subsequent preparations. For example, immersion cryofixation is excellently suited for freezing thin "sandwich" preparations and ultra-fine liquid films in grid meshes (the bare grid method), for which the metal mirror system is unserviceable.
On the other hand, by impacting animal or vegetable organs and tissues against a deeply cooled metal mirror, optimally plane surfaces are obtained, since the existing surface relief is considerably flattened by the impact. Surfaces produced on the metal mirror are therefore ideally suited for the subsequent preparation of sections on the cryo-ultramicrotome, whereas after immersion cryofixation tissues are only suitable for such use with substantial limitations.
It is therefore extremely inconvenient and costly in practical work, that in the majority of cases two different expensive instruments are required which must, in many respects, include similar parts for cooling, for the measurement and regulation of temperature, for the injection of specimens and for the subsequent cryo-transfer of the frozen specimens to an electron microscope or to apparatus for further processing, such as subsequent preparation in freeze-breaking and freeze-etching equipment, a cryo-substitution system or a cryo-ultramicrotome.